Step into nukage pool and flip the switch to fill up the pool. You will lose some health. When the pool is filled enough, pass through the alcove to enter a computer room from where the nukage was pouring. Get the radiation suit and super shotgun upstairs. Step into the northern area to the outside. Flip the switch that is south of this area. This unlocks the area to the east but it is too early to go there right now. Using the #-shaped structure, get into the northern hallway. Turn left and go westward into a hallway leading south. Enter a room with several blue torches and keep going on the linear way killing Hell knights. Get the red skull key on a pedestal and be careful as a trap will open behind you. Now you can backtrack to the nukage outside area.

Yellow key

When you get back to the area with #-shaped structure head to the eastern area through any alcove (destroy the barrels first). If this area is still inaccessible, flip the switch in the south of this area. In the eastern area, you can see four metallic structures with the yellow key inside the last one. First of all, before continuing your way through the red door, destroy the barrels between these structures as they will block your way when you will jump in there. After that, enter the red door and go up the stairs. Now keep going around the central area to a switch requiring the yellow key. Enter an area to the south in this corridor. All you need to do there is press a switch in the east. Get back into the corridor to the newly opened door leading to the central area. Make a run jump over several structures toward the westernmost one where the yellow key resides. If you have not yet destroyed the barrels underneath you will probably fall down, so do that and get back to repeat. When you finally get the key, you will need to unlock the exit.

Exit

Return through the red door and go around the corridor again. Flip the switch which is marked with yellow skull bars on the walls to unlock the exit. Get back to the #-shaped area and jump to the northern hallway again. Enter an area to the northeast. Four yellow skull bars are open now, so you can enter the exit teleporter.

At the center of the level is a thin roughly "#"-shaped walkway in a pool of nukage. In the northeast corner of the nukage pool, beneath a triangular outcropping of the wall, is a fake wall you can walk right through to get some hidden health bonuses. (sector 70)

After getting the red skull key, a wall behind you will open, revealing some enemies. One of the walls in the back of this newly revealed area can be opened, leading to a hidden radiation suit and a teleporter to a secret area. (sector 118)

At the last room (where you get ambushed by several imps, a baron of Hell and an arch-vile), in which you must press a switch to gain access to the yellow skull key, the middle of the three pillars can be lowered to reveal a teleporter that warps you to the otherwise inaccessible highest ovoid platform at the center of the "#"-shaped outer ledge.

When you press the switch to pour up the nukage in the starting area, quickly drop back to the starting teleporter. You can safely await while the nukage is pouring up and therefore save more health.

Upon acquiring the rocket launcher (or if you already have one), you can perform a death slide in the exit room by killing yourself with a rocket while running backwards toward one of the yellow skull bars, causing you to pass underneath them and onto the teleporter, ending the level. This was first demonstrated in Vincent Catalaá's 00:09 NM speed run of this level.